Background on Natural Attenuation

The intrinsic biodegradation of BTEX will result in a reduction in pollutant concentrations along the primary flow path downgradient from the source area. Such a reduction in pollutant mass is considered to be a direct or primary line of evidence for supporting natural attenuation as a groundwater remedy.

An indirect or secondary line of evidence which may be used to support the natural attenuation of BTEX is a correlation between the decrease in BTEX concentrations and a corresponding decrease in electron acceptor concentrations and an increase in metabolic byproduct (e.g., dissolved manganese, iron, or methane) concentrations.

Natural attenuation assessments involve an analysis of spatial and temporal trends in organic pollutant concentrations measured at groundwater monitoring wells. If pollutant concentrations are relatively stable or decreasing with time and if there is a significant reduction in concentrations downgradient from the source, these documented trends will provide primary evidence that intrinsic biodegradation is occurring at the site. Secondary lines of evidence may be required at some sites if the primary line of evidence is inconclusive.

An analysis of spatial and temporal trends based on tabulated data is a tedious undertaking and does not easily illustrate trends that support natural attenuation. Individual chemical contour maps may be difficult to interpolate because of site heterogeneity or at sites where source conditions are subject to significant temporal or spatial variability. It may be difficult to visualize a direct correlation between trends in organic pollutants and geochemical indicator concentrations by examining multiple contour maps. Also the preparation of contour maps is a subjective process because it is necessary to interpolate the chemical distribution between monitoring wells which is particularly difficult for sites which do not have a high-density monitoring network.

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